Cell cholesterol is of universal concern because of the enormous burden on health imposed by atherosclerotic cardiovascular disease. We now propose to continue our study o cellular cholesterol homeostasis. The hypothesis that the pool of cholesterol in the plasma membrane regulates its own abundance by signaling the endoplasmi reticulum (ER) through the regulated circulation of a stream of cholesterol will be tested. The set point of a putative cholesterol sensor in the plasma membrane will be characterized. The investigators will also test whether the cholesterol pool in the Golgi apparatus is regulated by the plasma membrane sensor; perhaps the Golgi serves as an intermediate in cholesterol transport. The flux of plasma membrane cholesterol through the lysosomes will be analyzed using cells from Niemann-Pick type C1 (NPC1) disease and cells treated with various amphiphiles which perturb cholesterol metabolism. The question of whether this movement represents specific transport or the flow of bulk plasma membrane bilayer will be addressed. The NPC1 gene product appears to be involved in cholesterol homeostasis. The investigators will analyze how its expression varies with cel cholesterol. Finally, the cell physiology of several sequenced mutants in NPC1 will be analyzed with respect to the gene defect to examine how NPC1 might function in cholesterol homeostasis.